Oil and gas reservoirs are accessed with a well casing extending downhole to a subterranean formation, and traversing various strata therealong. In the completion process, an annulus is formed between the casing and the formation. The annulus is filled with cement to seal the annulus, blocking cross-strata fluid communication and communication to surface. At the end of the commercial life of the well the well is abandoned.
The Alberta Energy Regulator currently requires that a “bridge plug” be installed as the first step in well abandonment. The bridge plug comprises a mechanical tool having a body carrying slips to grip the casing and an expandable, elastomeric seal ring to seal against the casing's inner surface. The tool can be operated by a tubing string extending down from surface. The body and seal ring thereby combine to permanently close and seal the cased bore.
During a conventional abandonment procedure the bridge plug is positioned and set at a pre-determined depth in the bore of the casing. A hydraulic pressure test is then carried out to determine if the bridge plug and casing are competent to hold pressure. The pressure test is currently performed by filling the casing bore with water and applying pressure at 1000 psi for 10 minutes. After it has been determined that both the bridge plug and the casing above the bridge plug are competent, a column of cement (typically 25 feet in length) is deposited in the bore immediately above the bridge plug. Finally, the top end of the steel casing is cut off at a point below ground level and a steel plate or vented cap is welded on the upper end of the casing.
However, problems can commonly arise over time with this system for plugging and abandoning wells. For example, the elastomeric element of the bridge plug may develop surface cracks or otherwise deteriorate and allow fluid to leak thereby.
Further, in the instance where the casing-to-cement and cement-to-formation seal fails, unacceptable hydrocarbon flow can occur to surface. Minute cracks may also develop in the cement column, including shrinkage of the cement sheath around the outside of the casing forming a micro-annulus where the cement abuts the inside surface of the casing. One or more of these defects can result in natural gas or other fluid leaking either up through the cased bore or along the outside surface of the casing to surface. Such leakage indicates that the abandonment process has failed. This failure is commonly identified when vegetation surrounding the well at ground surface begins to die from hydrocarbon exposure.
Presently there are thousands of wells in Alberta that have been abandoned. Many have been identified as leaking fluid to ground surface. Therefore, there is a need in the industry for an abandonment tool and method for closing and sealing wells which addresses the limitations of the current methods.